U.S. Pat. No. 5,114,660 discloses a method and system for the injection molding of plastic articles in an injection molding system including a pneumatically-operated gas compression unit having a high pressure gas receiver. A fluid pressure reducing valve, together with a directional control valve, controlled by a controller reduces the pressure of the high pressure nitrogen gas and communicates the pressurized fluid from the gas receiver to the injection molding system.
Briefly, gas-assisted injection molding is a thermoplastic molding process which provides stress-free large parts with a class A surface and virtually no sink marks. Gas-assisted injection molding is a low pressure molding process compared to conventional injection molding. In this process, inert gas such as nitrogen is injected into the plastic after it enters a mold. By controlling the gas pressure, the quantity of plastic injected into the mold (short shot) and the rate of gas flow, a predetermined network of hollow interconnected channels is formed within the molded part. The gas pressure remains constant in the network of hollow channels during the molding. This compensates for the tendency of the plastic to shrink at the thicker areas of the molding preventing warpage and reducing stress. The gas pressure is relieved just prior to opening the mold. Because of the relatively low injection pressure, large parts can be molded with substantial reductions in clamp tonnage.
Consequently, gas supply equipment must provide precise control of pressure, timing and volume of gas which is injected into the part, all of which are important to the control of the gas-assisted injection process.
In general, prior art valve assemblies exhibit relatively slow response and have a considerable amount of on-board electronics and take up a considerable amount of room, especially when multiple valve assemblies are required to service a number of injection molds or parts of molds.